The CsI photodetector
photodetector, filled with pure methane at ambient temperature and pressure, is
closed on one side by an end-flange where six independent CsI photocathode
boards of size 64x40 cm2, segmented into
pads of 8x8.4 mm2, are installed. On the opposite side, a honeycomb
panel supports three C6F14 radiator vessels placed at a
distance of 80 mm from the photocathode plane.
gas tightness is ensured by soft O-rings placed in grooves in the chamber
A view of the HMPID module-1 during HV tests
assembly procedure of module elements ensures the wire chamber gap (2 mm
anode–cathode distance) with a precision of 50 mm
and the parallelism between the radiator trays and the photon detector to better
than 100 mm.
MWPC anode plane is made of gold-plated
with 4.2 mm pitch, soldered on a G-10 printed board with a precision of 0.1 mm
and a tension of 47 g. On both edges of the anode plane, thicker guard wires are
installed to resist the boundary discontinuity of the electrostatic field.
At half-length of the module an insulating support structure is placed between
the pad cathode and the anode plane in order to minimize the sag of the sensing
wires due to the electrostatic force.
and collection wire planes are made out of 100 mm
diameter gold-plated Cu/Be wires, stretched at 200 and 50 g/wire, respectively,
by using crimping pins. A
positive voltage of 2050 V applied to the anodes, while cathodes are grounded,
provides a total gas gain of 8x104, allowing the detection of
ionizing particles and Cherenkov photons with single e- detection
efficiency of 90%.
The anode wires and the
Macor support lines
pad cathode is built by gluing together two printed circuit boards to achieve a
stiff and flat plane made out of light materials. The photosensitive layer is
obtained by depositing 300 nm of CsI onto the Cu-clad printed circuit boards,
electrolytically coated by 7 mm
thick Ni layer followed by a 0.5 mm thick Au layer and accurately pre-polished
through mechanical and chemical treatments.
The photocathode pad plane
photocathodes are prepared at CERN in the large evaporation station, equipped
with four DC heated tungsten crucibles operated simultaneously to achieve a
uniform CsI layer.
The CsI photocathode coating station